The invention relates to a magnetic disc device for use as an auxiliary storage or memory means for computers and to a method of correcting erroneous data pertinent to such disc device.
Automated electronic apparatuses including personal computers (PC) and office computers have been down sized and functionally enhanced very quickly supported by a recent progress in technology, which has in turn prompted development of compact yet fast and large-capacity memory devices or file systems such as a 5.2 inch, 3.5 inch, and 2.5 inch hard disc drives.
An effort has been paid to improve the density of data recorded on a magnetic disc and to increase the rotational speed of the spindle of the magnetic disc, to thereby improve the data transfer rate to and from a magnetic disc device. It is known that the level of the data which is reproduced or read from the magnetic disc by a magnetoresistive (MR) head does not depend on the linear speed or the rotational speed of the disc, so that the MR head can maintain a high signal level while reading data. Therefore, the MR head has become increasingly more important in this field.
It should be noted, however, that the MR head is maintained afloat while it is reading data only 30-50 nm above the magnetic medium due to a negative pressure acting on the MR head. Prior to read operation, the magnetic head is at rest on the surface of the magnetic disc, which is also at rest. If the magnetic disc has a very flat surface like a mirror, the head would stick to the surface of the disc and would remain captured by the magnetic disc even after the magnetic disc had started its rotation. In order to prevent such capture, the magnetic disc is provided on the surface thereof with minute protrusions formed by an appropriate surface roughening or texturing technique.
However, such roughening process can form a small number of unexpectedly large protrusions on some tracks on the magnetic disc. The magnetic head will then bump on such large protrusions every time the head passes over the tracks. Collisions of the head with the magnetic disc can also take place when the magnetic disc is heated during read/write operations and the surface of the disc gets deformed, or when the head is thermally deformed, or when the entire magnetic device is subjected to a strong mechanical shock during the operations.
When such a collision takes place, the MR head is temporarily heated for a few microseconds. The resistance of the heated MR head is changed accordingly.
During a read operation the MR head undergoes a relative motion over the magnetic disc while passing a constant electric current through the magnetoresistive element thereof. The magnetoresistive element changes its resistance if a magnetic field externally applied thereto is changed. Hence, the MR head can read magnetically recorded information on the disc in the form of the voltage change across the magnetoresistive element by detecting the change in resistance of the magnetoresistive element. Therefore, if the head collides with a bump or protrusion formed on the disc, the head is temporarily heated and its resistance is changed. This results in a corresponding change in voltage and errors in the information reproduced (i.e. read) from the magnetic disc. Such transient voltage change (which lasts for a few microseconds) can result in erroneous data as much as several tens of bytes.
When such a transient thermal event as mentioned above occurs, there will be a transient waveform or a DC bias in the output of the head, which smears the data read from the disc, and makes the data unrecoverable in a subsequent decoding stage, thereby resulting in unrecoverable read errors. This type of errors observed in MR heads is called TA or thermal asperity.
A strategy well known in the art to avoid thermal asperity is to use a high pass filter (HPF) in a data decoding circuit to cut off low-frequency components of the signal reproduced so as to suppress generation of a transient waveform and promote quick convergence of the disturbed signal. An alternative approach known in the art to hold the reproduced signal is to provide either an automatic gain control (AGC) circuit which causes the data decoding circuit to hold the amplitude of the reproduced signal for a moment or a phase locked loop (PLL) circuit which temporarily prohibits the reproduced signal to follow the DC bias and become out of phase with a clock.
A further technique is known to deal with errors that cannot be corrected by the circuits as mentioned above, which is adapted to recover correct data by means of a circuit, called ECC circuit, provided in the decoding circuit for executing so-called error correction codes (ECC) on the fly.
However, most recent magnetic disc devices have much higher data transfer rates than conventional ones that the amount of data that will be lost due to thermal asperity if it occurs often exceeds the data length that can be corrected by the ECC circuit.
For example, the data length correctable by an ECC circuit is estimated to be at most about 9 bytes for an average magnetic disc device, and at most 20 bytes for a most advanced magnetic disc device. The duration of a thermal asperity is of order of a few microseconds, which corresponds to a loss of data of about several tens bytes, which exceeds the error correction ability of the today""s ECC circuit, and hence the ECC circuit cannot deal with such errors.
It is, therefore, an object of the invention to provide an erroneous data correction device which may determine the location and the length of errors in the data reproduced or read from a magnetic recording medium.
It is another object of the invention to provide a method which may determine the location and the length of errors in the data read from a magnetic recording medium.
It is a further object of the invention to provide a magnetic disc device including an MR head having an MR element for reading data from the magnetic recording disc, the magnetic disc device equipped with means for detecting read errors introduced in the data by a sudden change in electric resistance of the MR element caused by a short collision of the MR head with the magnetic recording disc.
It is a still further object of the invention to provide a magnetic disc device which is equipped, in addition to means for detecting read errors mentioned above, with data recovery means for recovering the data lost by the errors.
It is a still further object of the invention to provide a method for detecting erroneous data involved in the foregoing magnetic disc device.
It is a still further object of the invention to provide a method for detecting and correcting erroneous data involved in the foregoing magnetic disc device.
In accordance with one aspect of the invention there is provide an erroneous data correction device, comprising:
a comparator which compares a first signal indicative of data read from a magnetic recording medium (referred to as data or data signal) and input thereto with a second signal having a predetermined threshold level (referred to as threshold signal) input thereto, for generates an output when the level of the data signal exceeds the threshold signal; and
an error signal generation circuit for generating an error signal using the output of the comparator and the data signal.
This error signal generation circuit device may include means for determining the location of erroneous data included in the data signal, using the error signal.
The erroneous data includes a transient waveform generated mainly by thermal asperity.
In the erroneous data correction device, the error signal generation circuit is adapted to select either one of the output of the comparator circuit, the data signal obtained from the magnetic recording medium, or a logical sum of the data signal and the comparator output, and outputs the selected signal as the error signal.
The erroneous data correction device may further include a counter circuit which has input ends for receiving the error signal and a read clock signal. The counter counts up read clock from the beginning of the data. The location of an erroneous data in the data may be obtained from the count up to the occurrence of an error signal. In addition, the erroneous data correction device may further include a register circuit connected with the counter circuit.
The erroneous data correction device may determine the length of the erroneous data included in the data based on the error signal.
The erroneous data correction device may have a further counter which receives the error signal from the error signal generation circuit and a read clock signal from a clock and counts the read clock over the period that the error signal is generated. Thus, the length of the erroneous data may be determined from the count.
An alternative error signal may be generated by another means of the invention which includes
a data reproduction circuit for reproducing data read from the magnetic recording disc; and
a decoder for use in the data reproduction circuit for performing a data format transformation on the bits of the reproduced data in accordance with a given set of run length rules to generate an error signal.
In this case the decoder may employ zero run-length rules in the form of 8/9 (0, n/m) data format.
In accordance with another aspect of the invention, there is provide an data error correction device, comprising:
a comparator which compares a first signal indicative of data read from a magnetic recording medium (data signal) and input thereto with a second signal having a predetermined threshold level (threshold signal) input thereto, for generating an output when the level of the data signal exceeds the level of the threshold signal; and
an error signal generation circuit for generating a first error signal using the output of said comparator and said data signal;
a decoder for use in the data reproduction circuit for performing a data format transformation on the bits of the reproduced data;
means for generating a second error signal based on the run-length rules imposed on said reproduced data; and
means for selecting either one of the first and the second error signals.
A magnetic disc device may include any of the erroneous data correction devices as described above.
In a still further aspect of the invention, there is provided a magnetic disc device, comprising:
a decoding circuit for decoding signals using partial response maximum likelihood technique;
a comparator for generating an output signal when the level of the data signal reproduced from the magnetic disc exceeds a predetermined threshold level;
means for determining the location and the length of erroneous data included in the data decoded by the decoder using the output of the comparator and decoded data;
hardware means for executing hardware ECC on-the-fly on the erroneous data; and
software means for executing software ECC on the erroneous data; and
means for choosing the hardware ECC correction means when the length of the erroneous data is longer than the length correctable by the hardware means, but otherwise choosing the software means.
In a still further aspect of the invention, there is provided a magnetic disc device, comprising:
a decoding circuit for decoding signals using partial response maximum likelihood technique;
a decoder for use in the data reproduction circuit for performing a data format transformation on the bits of the reproduced data;
means for generating an error signal based on the run-length rules imposed on said reproduced data;
means for determining the location and the length of the erroneous data based on the error signal output from the error signal generation means and the data decoded by the decoding circuit using partial response maximum likelihood technique;
hardware means for executing hardware ECC on-the-fly on the erroneous data; and
software means for executing software ECC on the erroneous data; and
means for choosing the hardware ECC correction means when the length of the erroneous data is longer than the length correctable by the hardware means, but otherwise choosing the software means.
In a still further aspect of the invention, there is provided an error correction method comprising steps of:
comparing a data signal reproduced from a magnetic disc with a predetermined threshold signal to generate an output signal when the level of the data signal exceeds the level of the threshold signal;
generating an error signal associated with the erroneous data included in the data signal based on the output signal and the data signal; and
determining the location and the length of the erroneous data based on the error signal.
In a still further aspect of the invention, there is provided an error correction method of the invention, comprising steps of:
determining the location and the length of erroneous data included in data signal reproduced from a magnetic disc;
determining whether said erroneous data has a length correctable by ECC on the fly;
executing ECC on the fly on the erroneous data when ECC on the fly is applicable; and
otherwise executing software ECC on the erroneous data.